1. Field of the Invention
The invention relates to a vacuum pressure swing adsorption process and system for the production of oxygen. More particularly, it relates to a process and system adapted for the large scale production of oxygen from air.
2. Description of the Prior Art
In numerous chemical processing, refinery, metal production and other industrial applications, purified gas streams are employed for a variety of processing purposes. For example, high purity oxygen is used in chemical processing, steel mills, paper mills, and in lead and gas production operations. Oxygen and nitrogen are produced from air, typically by cryogenic distillation for large size applications. While such cryogenic processing can be very efficient, particularly when conducted in large size plants, it nevertheless requires complex and costly equipment.
Pressure swing adsorption (PSA) processes have also been used to separate and purify gases, but the production of oxygen by the PSA approach has generally been confined to relatively small-sized operations with respect to which the use of cryogenic air separation may not be economically feasible. Many commonly available adsorbents, particularly the class of materials known as molecular sieves, selectively adsorb nitrogen more strongly than oxygen, and this preferential adsorption is the basis of a variety of PSA processes that have been developed for the separation of air to produce oxygen and nitrogen product gas.
PSA processes for air separation to produce product oxygen are known in the art, as indicated by the Skarstrom patent, U.S. Pat. No. 2,944,627. Such processes typically comprise four separate operating steps carried out, in turn, in each bed of multi-bed PSA systems. Such steps are (1) adsorption, in which feed air is passed at an upper adsorption pressure to the feed end of the bed, containing adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component of air, with the less readily adsorbable oxygen being recovered from the product end of the bed; (2) countercurrent depressurization at a lower desorption pressure; (3) desorption of the more readily adsorbable nitrogen from the adsorbent bed, and its withdrawal from the feed end of the bed with or without the introduction of purge gas to the product end of the bed; and (4) repressurization of the bed to the upper adsorption pressure. This processing sequence, or variations thereof, is then repeated in each bed of the system with additional quantities of feed air, as continuous oxygen-producing operations are carried out in the PSA system.
When the PSA process is employed principally to remove strongly adsorbable impurities present in low concentration in a gas stream, i.e. CO.sub.2 and/or H.sub.2 O in air, the constant pressure steps of adsorption (1) and desorption (3) occupy most of the processing cycle time, and the pressure change steps, i.e. countercurrent depressurization (2) and repressurization (4) are transients. In the production of oxygen from air, where the more readily adsorbable nitrogen comprises 79% of the feed stream, the pressure change steps are of greater significance to the overall processing efficiency. Many different modifications of the basic PSA processing sequence have been developed, including many variations in the pressure swing steps.
Most PSA processes for producing oxygen from air are carried out in multi-bed system, i.e. systems incorporating two or more adsorbent beds, with each bed undergoing the same sequence of steps, but in a different phase relationship with the other beds in the system. The processing steps are synchronized and are usually carried out for a fixed period of time. Operating in this manner, the supply of oxygen product can be made more steady, and the utilization of mechanical pumps made more nearly constant than otherwise would be the case. Many PSA processes also employ one or more pressure equalization steps, wherein gas withdrawn from one bed at high pressure is passed to another bed initially at low pressure until the pressures in said beds are equalized. This procedure has the advantages of saving some compression energy, supplying the equivalent of purge gas if the equalization is accomplished through the product ends of the beds, and elevating the overall recovery of desired oxygen product.
A particular PSA process for producing oxygen from air utilizes a three bed system and incorporates the following processing steps: (1) adsorption with feed air introduction, bed pressurization and simultaneous oxygen product recovery; (2) co-current depressurization for further product recovery; (3) pressure equalization; (4) countercurrent depressurization; (5) purge and (6) repressurization. This process operates with a typical upper adsorption pressure of 50 psia, and a lower desorption pressure of one atmosphere. While this processing system and process can effectively recover oxygen from air, it is not sufficiently efficient for use in large volume commercial operations. The operating costs for such systems are high because of the relatively high compression ratio required. For a given product flow rate, the adsorbent inventory required for such systems is also relatively high.
PSA processes have also been developed that operate between atmospheric adsorption pressure and a relatively deep vacuum desorption pressure. Since the adsorptive storage of nitrogen is pressure dependent, such processes and systems, i.e. vacuum pressure swing adsorption (VPSA) processes and systems, require a large adsorbent inventory, which greatly increases the capital costs associated therewith.
It has also been found that this process can be improved by employing a partial pressure-equalization step instead of the essentially full pressure equalization conventional in the PSA art in which the adsorbent selectively adsorbs nitrogen from air. Various other modifications of the basic PSA process have been proposed in the art, with most being related to variations in the pressurization and depressurization steps. Suh and Hankat, in AICHE J 1989 35 523, have, for example, reported on the merits of using combined co-current-countercurrent depressurization steps in PSA processing. For producing oxygen from air, they report findings that the addition of a simultaneous co-current depressurization step is not helpful. Their two bed cycle utilizes a backfill repressurization step, wherein the product end of the high pressure bed is connected to the product end of the low pressure bed, with passage of gas from one bed to another being continued until the lower pressure bed attains the high pressure.
Kiow and Kenny, AICHE J (1990) 36 53, have also studied a backfill step for oxygen production. Applying a mathematical model that includes rate effects as well as the equalization properties of the adsorbent, i.e. 5A zeolite, to the behavior of a super-atmospheric PSA cycle incorporating such a backfill step, with the flow rates being controlled. They found that this PSA processing cycle was beneficial for producing an enriched oxygen product. The maximum oxygen purity reported, however, was less than 80%, which is much less than the oxygen concentration required for a high purity oxygen product.
Current VPSA technology is constrained by limited economics of scale at plant sizes above 125 TPD oxygen production. Commercial VPSA systems usually employ positive displacement "Roots" type compression elements, which reach the limit of standard commercial equipment at this plant size. Machines of larger size would generally be custom designed and very expensive. The logical course would appear to be to use centrifugal compression elements. However, the efficient use of centrifugal type compressors is made difficult since VPSA processing cycles require that the compression elements operate under variable head conditions.
When large capacity plants are required, multiple systems are usually employed because of the limitations referred to above at above 125 TPD oxygen production. The use of multiple systems results, however, in nearly doubling the cost of the plant when the size is doubled. Present commercial systems are typically two or three bed systems. Such systems are single compressor designs employing similar components, and, therefore, are subject to similar economics and size limitations.
Other processing and systems for large scale oxygen production have been proposed, such as a four bed refrigerated cycle employing multiple compressors and adsorbers in large plant facilities. This cycle employs four full flow adsorbers that are quite large and have demonstrated advantage over conventional two or three bed cycles. The compression and vacuum equipment for this cycle are also sized for full flow with some indexing of the vacuum equipment as required to achieve the appropriate vacuum level. In addition, this cycle results in complex piping systems, and adsorbers and compression equipment that are large and costly. Furthermore, the pressure ratio applicable to such a refrigerated cycle does not allow for the national implementation of centrifugal equipment into the system.
It is apparent that a great many modifications and variations of the VPSA cycle, or processing sequence, have been investigated, with many such modifications or variations having been employed in commercial VPSA operations, as for the production of oxygen from air. A wide variety of possible individual steps for carrying out the pressurization and depressurization steps have been investigated. In spite of such extensive efforts, conducted over a long period of time, VPSA processes for the production of high purity oxygen remain inefficient and uneconomical, especially for large plant applications. Thus, there remains a need in the VPSA air separation field for still more efficient VPSA processing that can be scaled up for the large volume production of high purity oxygen from air.
It is an object of the invention, therefore, to provide an improved, highly efficient VPSA process and system for producing oxygen from air.
It is a further object of the invention to provide an improved PSA process and system having lower power consumption and lowered capital costs than those pertaining to conventional commercial PSA processes for the production of large volumes of high purity oxygen from air.
With these and other objects of the invention in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.